1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic apparatus, electrostatic printer, etc., and in particular to an image forming apparatus for transferring an image which is obtained by developing an electrostatic latent image to a transfer material, such as a paper sheet, by a transfer device.
2. Description of the Related Art
An image forming apparatus, such as an electrophotographic apparatus and electrostatic printer, is adapted to form an electrostatic image on a photoconductive drum (image carrier), electrostatically attract a developing agent to the electrostatic latent image to provide a developed image and transfer the developed image to a paper sheet so that recording is carried out.
An electrostatic transfer device using a corona transfer method or roller transfer method, as well as a mechanical transfer device using an adhesive transfer method, etc., are known as the transfer device in this field of art.
After transfer has been effected, there remains an electrostatic latent image and untransferred developing agent on the photoconductive drum. The remaining developing agent is removed by a cleaner. Then, the electrostatic latent image is erased by a discharger. In this way, the above-mentioned operations are repeated.
Recently, a growing demand has been made for more and more compact image forming apparatus. The generation of ozone caused upon corona discharge poses a problem harmful to the human being when the electric transfer device is employed. For this reason, the transfer device using the roller transfer method is desirable because it generates less ozone.
In spite of that major advantage of the roller transfer method, there are several reasons for which this method has generally not been employed.
For the roller transfer method it is necessary that a transfer material, such as a paper sheet, be pressed against the image forming surface of the photoconductive drum by the transfer roller under proper pressure and applied voltage. If the pressure is insufficient, uneven transfer is encountered. Under excessive pressure, on the other hand, a developing agent (toner) is adhered to the image forming surface of the drum so that an image transfer is partially skipped on the paper sheet. It is, therefore, necessary to manufacture that transfer roller with high mechanical accuracy (straightness about 50 microns) and proper pliability (JIS hardness of about 10.degree. to 40.degree.). However, electroconductive rubber used as a material for the conventional transfer roller can hardly achieve the above two requirements.
Specifically, if a paper sheet as thick as 100 microns is used, a defective image transfer occurs owing to the generation of excessive pressure involved. In this case, it is necessary to perform complex control wherein the transfer roller is moved toward and away from the photoconductive drum so as to enable the transfer roller to meet the thickness of the paper sheet.
It is generally also necessary to exert an electrostatic force on the developed image so that, subsequent to transferring the developed image to the paper sheet, the resultant paper sheet is separated from the transfer roller. In order to generate such an electrostatic force, the transfer roller has a predetermined electrical resistance. It is required that, under all the circumferences, the aforementioned resistance be so maintained as to prevent a breakdown of a recording medium by the electrostatic force. This restricts the range in which the material for the transfer roller is selected.
In order to solve the aforementioned problems under these backgrounds, there is a demand for an image forming apparatus adequate enough to satisfy these required characteristics.